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diskimage-builder/doc/source/developer/vhd_creation.rst
Ian Wienand 6758849b66 vhd-util : note on Xen/RAX images 
Add a note covering what we've discovered about creating RAX/Xenserver
compatible images over the last few days.

Change-Id: Iffd9dab1ca54f27390ec7850093e828ca7576e98
2019-10-31 08:36:07 +11:00

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vhd creation
============
There is currently a hard dependency on the patched version of
``vhd-utils`` to create Xenserver VHD compatible images. The main
target for these images is the Rackspace Public Cloud.
This note should explain the current issues.
The patch to include ``convert`` as an option to ``vhd-utils``,
avaialble at https://github.com/citrix-openstack/xenserver-utils/, is
built by infra into a PPA available at
https://launchpad.net/~openstack-ci-core/+archive/ubuntu/vhd-util.
Upstream Xen has actually removed all the code related to ``blktap2``
with
https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=5c883cf036cf5ab8b1b79390549e2475f7a568dd.
The tools have been removed from Debuntu with
https://bugs.debian.org/917907. So this patch is even less trivial to
apply; the project has seemingly been split into
https://github.com/xapi-project/blktap/ but support status is unclear.
It is tempting to use ``qemu-img`` image conversion routines. Indeed
this supports converting RAW images to VPC output. However, VPC is a
fairly flexible standard across a number of different hypervisors.
The main standard is available from Microsoft at
https://www.microsoft.com/en-us/download/confirmation.aspx?id=23850
and is what ``qemu-img`` impelements.
However, trying to boot an image created with this results in a
"resize" error. This is described in
https://bugs.launchpad.net/nova/+bug/862653 but comes down to the
following in
https://github.com/xapi-project/blktap/blob/master/vhd/lib/vhd-util-resize.c#L1055::
if (!vhd_creator_tapdisk(&vhd)) {
printf("%s not created by xen; resize not supported\n", name);
err = -EINVAL;
}
It seems that Xen will refuse to grow a image without a creator
``tap\0``. It is tempting to patch ``qemu-img`` to do this (patch
follows for those interested). However, if you look at the output
with ``vhd-util read -p -n ./<file>.vhd`` you will notice it complains
about an invalid ``batmap``. The ``batmap`` appears to be a Xen
extension to the VPC standard. Checking ``vhd_has_batmap`` at
https://github.com/xen-project/xen/blob/365aabb6e5023cee476adf81106729efd49c644f/tools/blktap2/vhd/lib/libvhd.c#L1193
it seems we can fool Xen by setting the vhd version tags low enough
that it thinks the image doesn't have a ``batmap`` (i.e. presumably
created with tools before ``batmap``'s existed).
Side note: there is a bunch of qemu work to implement overlay VHD
disks with
https://patchwork.ozlabs.org/project/qemu-devel/list/?submitter=64750
which has never merged. This also seems to include a ``batmap``
implementation.
With the full patch applied below, we get images that appear fairly
similar from (patched) ``vhd-util`` and ``qemu-img``; generated from
the same ``.raw`` file.
qemu-img output::
VHD Footer Summary:
-------------------
Cookie : conectix
Features : (0x00000002) <RESV>
File format version : Major: 1, Minor: 0
Data offset : 512
Timestamp : Wed Oct 30 06:25:41 2019
Creator Application : 'tap'
Creator version : Major: 1, Minor: 0
Creator OS : Unknown!
Original disk size : 16667 MB (17477591040 Bytes)
Current disk size : 16667 MB (17477591040 Bytes)
Geometry : Cyl: 33865, Hds: 16, Sctrs: 63
: = 16667 MB (17477591040 Bytes)
Disk type : Dynamic hard disk
Checksum : 0xffffec8c|0xffffec8c (Good!)
UUID : 1bfda481-dd4d-43aa-8f3a-84689b5ab3d7
Saved state : No
Hidden : 0
VHD Header Summary:
-------------------
Cookie : cxsparse
Data offset (unusd) : 18446744073709
Table offset : 1536
Header version : 0x00010000
Max BAT size : 8334
Block size : 2097152 (2 MB)
Parent name :
Parent UUID : 00000000-0000-0000-0000-000000000000
Parent timestamp : Sat Jan 1 00:00:00 2000
Checksum : 0xfffff3c9|0xfffff3c9 (Good!)
vhd-utils output::
VHD Footer Summary:
-------------------
Cookie : conectix
Features : (0x00000002) <RESV>
File format version : Major: 1, Minor: 0
Data offset : 512
Timestamp : Tue Oct 8 09:18:54 2019
Creator Application : 'tap'
Creator version : Major: 1, Minor: 3
Creator OS : Unknown!
Original disk size : 16668 MB (17477664768 Bytes)
Current disk size : 16668 MB (17477664768 Bytes)
Geometry : Cyl: 33865, Hds: 16, Sctrs: 63
: = 16667 MB (17477591040 Bytes)
Disk type : Dynamic hard disk
Checksum : 0xffffeebb|0xffffeebb (Good!)
UUID : 14f2710a-b9b8-48f7-94bb-005ba6d566b2
Saved state : No
Hidden : 0
VHD Header Summary:
-------------------
Cookie : cxsparse
Data offset (unusd) : 18446744073709
Table offset : 1536
Header version : 0x00010000
Max BAT size : 8334
Block size : 2097152 (2 MB)
Parent name :
Parent UUID : 00000000-0000-0000-0000-000000000000
Parent timestamp : Sat Jan 1 00:00:00 2000
Checksum : 0xfffff3c9|0xfffff3c9 (Good!)
VHD Batmap Summary:
-------------------
Batmap offset : 35840
Batmap size (secs) : 3
Batmap version : 0x00010002
Checksum : 0xfffbf214|0xfffbf214 (Good!)
This does upload and boot in RAX, but the root disk does not appear to
grow correctly.
Emperically, it also seems that fixed sized disks (generated with
``subformat=fixed``) will not import (at least into Rackspace). I
also tried resizing the dynamic ``vhd`` (with ``vhd-util
resize --debug -n ./test.vhd -s $((32 * 1024 * 1024 )) -j
resize.log``), which looked correct in ``vhd-util`` output (``Current
disk size`` grew) but Rackspace would not import this image.
qemu patch
==========
Applies against qemu at HEAD
``16884391c750d0c5e863f55ad7aaaa146fc5181e``
::
diff --git a/block/vpc.c b/block/vpc.c
index a655502..d1716f0 100644
--- a/block/vpc.c
+++ b/block/vpc.c
@@ -60,6 +60,7 @@ enum vhd_type {
#define VHD_MAX_GEOMETRY (VHD_CHS_MAX_C * VHD_CHS_MAX_H * VHD_CHS_MAX_S)
#define VPC_OPT_FORCE_SIZE "force_size"
+#define VPC_OPT_XENSERVER_COMPAT "xenserver_compat"
/* always big-endian */
typedef struct vhd_footer {
@@ -1042,12 +1043,16 @@ static int coroutine_fn vpc_co_create(BlockdevCreateOptions *opts,
memset(buf, 0, 1024);
memcpy(footer->creator, "conectix", 8);
- if (vpc_opts->force_size) {
+ if (vpc_opts->xenserver_compat) {
+ memcpy(footer->creator_app, "tap\0", 4);
+ memcpy(footer->creator_os, "\0\0\0\0", 4);
+ } else if (vpc_opts->force_size) {
memcpy(footer->creator_app, "qem2", 4);
+ memcpy(footer->creator_os, "Wi2k", 4);
} else {
memcpy(footer->creator_app, "qemu", 4);
+ memcpy(footer->creator_os, "Wi2k", 4);
}
- memcpy(footer->creator_os, "Wi2k", 4);
footer->features = cpu_to_be32(0x02);
footer->version = cpu_to_be32(0x00010000);
@@ -1058,9 +1063,14 @@ static int coroutine_fn vpc_co_create(BlockdevCreateOptions *opts,
}
footer->timestamp = cpu_to_be32(time(NULL) - VHD_TIMESTAMP_BASE);
- /* Version of Virtual PC 2007 */
- footer->major = cpu_to_be16(0x0005);
- footer->minor = cpu_to_be16(0x0003);
+ if (vpc_opts->xenserver_compat) {
+ footer->major = cpu_to_be16(0x0001);
+ footer->minor = cpu_to_be16(0x0000);
+ } else {
+ /* Version of Virtual PC 2007 */
+ footer->major = cpu_to_be16(0x0005);
+ footer->minor = cpu_to_be16(0x0003);
+ }
footer->orig_size = cpu_to_be64(total_size);
footer->current_size = cpu_to_be64(total_size);
footer->cyls = cpu_to_be16(cyls);
@@ -1101,6 +1111,7 @@ static int coroutine_fn vpc_co_create_opts(const char *filename,
static const QDictRenames opt_renames[] = {
{ VPC_OPT_FORCE_SIZE, "force-size" },
+ { VPC_OPT_XENSERVER_COMPAT, "xenserver-compat" },
{ NULL, NULL },
};
@@ -1220,6 +1231,11 @@ static QemuOptsList vpc_create_opts = {
"specified, rather than using the nearest CHS-based "
"calculation"
},
+ {
+ .name = VPC_OPT_XENSERVER_COMPAT,
+ .type = QEMU_OPT_BOOL,
+ .help = "Set creator to tap"
+ },
{ /* end of list */ }
}
};
diff --git a/qapi/block-core.json b/qapi/block-core.json
index aa97ee2..68daa3c 100644
--- a/qapi/block-core.json
+++ b/qapi/block-core.json
@@ -4669,6 +4669,7 @@
# @force-size Force use of the exact byte size instead of rounding to the
# next size that can be represented in CHS geometry
# (default: false)
+# @xenserver-compat Xenserver comapt
#
# Since: 2.12
##
@@ -4676,7 +4677,9 @@
'data': { 'file': 'BlockdevRef',
'size': 'size',
'*subformat': 'BlockdevVpcSubformat',
- '*force-size': 'bool' } }
+ '*force-size': 'bool',
+ '*xenserver-compat': 'bool'
+ } }
##
# @BlockdevCreateOptions:
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